D-glucaric acid or saccharic acid is being widely used for preparing adipic acid by deoxydehydra-tion reaction. In turn, adipic acid is extensively used for manufacturing nylon-6,6. The glucaric acid is commercially available, but only as an aqueous solution. This limits the reaction solvent to water or aqueous mixtures. Alternatively, salts of D-glucaric acid have also been used for manufacturing adipic acid, to meet the growing demand. However, upon manufacturing adipic acid in the presence of the salts of D-glucaric acid, several intermediates are formed, which are difficult to separate out due to their tendency to exist as a mixture. Nevertheless, these intermediates, if obtained selectively, may turn out to be reactants for important industrial chemicals.
These important intermediates are lactones, which exist in the mono- and di-form. Examples of such mono-lactones include D-glucaro-1,4-lactone and D-glucaro-6,3-lactone while D-glucaro-1,4:6,3-lactone is one such intermediate di-lactone. Along with these mono- and di-lactones, glucaric acid is also formed which exists as an equilibrium solution with the lactones.
Of the above mentioned lactones, D-glucaro-6,3-lactone is an industrially important mono-lactone of D-glucaric acid. The commercial availability of D-glucaro-6,3-lactone is very limited or almost nil. The ability of this particular mono-lactone to be used for manufacturing various amino acids, amino esters and amino alcohols, has led to developments in arriving at various means for its selective production.
Acidification of calcium D-glucarate tetrahydrate with sulfuric acid in the presence of acetone-water is discussed by Troy et. al. [J. Org. Chem. 2009, 74, 8373-8376]. The acidification step is followed by filtration, reduced pressure operation and concentration steps to finally obtain a concentrated aqueous solution containing solid particles of a mixture of D-glucaric acid, D-glucaro-1,4-lactone, D-glucaro-6,3-lactone and D-glucaro-1,4:6,3-lactone in a fixed ratio.
Another study reported by Davey et. al. [Carbohydrate Research, 341 (2006), 2688-2693] discloses a mixture of D-glucaric acid lactones from a suspension of monopotassium glucarate in de-ionized water. During the synthesis, ion exchange resin was added, the filtrate was evaporated under reduced pressure and freeze dried to yield an amorphous solid containing a mixture of acyclic d-glucaric acid, D-glucaro-1,4-lactone, D-glucaro-6,3-lactone and D-glucaro-1,4:6,3-lactone in a fixed ratio.
Cation exchange resin was added to a mixture of monopotassium D-glucarate and water in a study conducted by Chen and Kiely [J. Org. Chem. 1996, 61, 5847-5851]. Acid form of cation exchange resin was added further with filtration and concentration carried thereafter. D-glucaro-6,3-lactone was obtained after 2-3 days of crystallization in the form of white solids and used for synthesis of head, tail hydroxylated nylons.
The existing techniques for selectively obtaining D-glucaro-6,3-lactone are not satisfactory in terms of low yield, purity of final product and time taken to obtain the final product through various process steps. Moreover, the relatively high yield and selectivity of undesirable products such as other mono- and di-lactones renders these techniques unfavorable. The requirement of high overall process temperature further renders these technique complex and uneconomical.
Thus, it was an objective of the presently claimed invention to provide a process for selectively preparing D-glucaro-6,3-lactone having high purity with process conditions which render the invention economical by optimizing them in a manner that the yield of D-glucaro-6,3-lactone is selectively maximized with minimum formation of other mono- and di-lactones as well as acid.